Francisella tularensis (Ft) is a gram-negative intracellular pathogen that causes a fatal disease known as tularemia. Based on its high virulence, use in bioweapon programs in the past, and potential to be used in bioterror attacks, the Centers for Disease Control (CDC) have classified Ft as a Category A select agent. The extreme virulence of Ft is due to its ability to subvert host's innate immune response. In support of this notion, our preliminary studies demonstrate that the virulent strain Ft SchuS4 and Live Vaccine Strain (LVS), a derivative of F. holarctica exert suppressive effects on innate immune signaling cascades resulting in very low levels of pro-inflammatory cytokines associated with significant delay in the programmed death of the infected cells. However, the mechanisms and the Francisella factors mediating these effects are ill defined. Two major classes of innate immune sensors, Toll-like receptors (TLRs) and cytosolic Nod-like receptors (NLRs) are activated in response to specific microbial components. Activation of these sensors leads to induction of pro- inflammatory cytokines, recruitment of innate immune cells to the sites of infection, generation of an adaptive immune response and resolution of infection. Recognition of pathogens by cytosolic NLRs activates assembly of a multi-molecular complex termed as inflammasome. The inflammasome complex activates cysteine protease caspase1 which in-turn activates pro-forms of pro-inflammatory cytokines IL-1 and IL-18 to their mature active forms. Inflammasome assembly and caspase-1 activation also induces a rapid form of pro- inflammatory cell death, termed pyroptosis. Our preliminary studies demonstrate that F. tularensis is capable of suppressing both TLR and NLR-mediated responses and pyroptotic cell death. This proposal is aimed at investigating the mechanism by which virulent strains of Ft suppress NLR-dependent responses. Our hypothesis is that active repression of the inflammasome is essentially required for intracellular survival of Ft. We propose that by repressing or delaying this protective host's innate immune response, Ft is able to create a replicative niche in the cytosol leading to uncontrolled proliferation within the host macrophages. This hypothesis will be tested via Specific aim 1 which will establish the key sites of repression of inflammasome by Ft LVS and SchuS4 while in Specific aim 2, we will identify the mechanisms of Ft mediated repression of inflammasome. Ft serves as a unique model to understand the intricate mechanisms of inflammasome mediated responses. The outcome from these studies will identify mechanism(s) employed by Ft to repress the inflammasome, a critical host innate immune defense mechanism. These studies will advance our knowledge of tularemia pathogenesis and will bring us closer to achieving the ultimate goal of identifying the targets for development of effective therapeutics and prophylactics. This proposal is a step ahead in understanding the immunopathogenesis of tularemia.